Shark Racer

Did you notice a decrease in engine braking? Driving around in first gear, the car hated to rev high. After switching to the duals, the car seems to stay above 4k much easier. Makes winding through a corner nicer. Did anyone else notice this from the switch?

What's the theory behind this?

-Steve

fauxrs

i noticed it - but have no answer
:lol:

Zane

I have dual exhaust and I noticed no diffrence. Must be that automatic tranny I'm using. :crazy:

Smokehouse69

I'd guess that the backpressure of the old single exhaust probably acts sort of like an exhaust brake that put on some diesel engines, to provide engine braking.
I remember the guys on the old Shadetree Mechanic show putting an exhaust brake on a Dodge diesel pickup, it worked by closing off the exhaust with a flapper.

Schmucker

Never drove one with single exhaust. Engine hits 7k with ease though, just in a corner the rear end tries to get ahead of me if I do that :D

LiveandLetDrive

Hey, I saw that episode! I'd agree with that theory, can't say me and my auto trans have noticed this though!

-Chris

Disco_Mike

I've had true duals on the old girl for more then 20 years. I can't remember how it drove with the stock exhaust. My father and I did the mod within the 1st month of ownership. :smash:

jerryp58

I believe the expression is "compression braking".

Off the top of my head, I believe it is the fact that: as the exhaust flow is restricted, so is the ability of the piston to move and push gas out of the exhaust port. The reverse is also true. So, if you "open up" the exhaust, it's easier for the piston to move & push the gas out which means the crank will turn easier, which means the car will roll easier.

I still have 2-1-2 on mine with 3" pipe as the single and 2-1/2" for the dual parts, but I still have a fair bit of compression braking ability. I've always liked the effect. The spouse has a 2000 Grand Am that somehow has NO compression braking ability and it's really weird to drive. I sometimes find myself hitting the brakes hard because the car didn't slow down like I expected when I took my foot off the gas. :eek:

Shark Racer

One volvo I drove, about 40 mph in first gear. "Why won't the thing go any faster?" Whoops, left it in first.

Felt smooth as hell, no compression braking whatsoever.

Not saying compression braking is gone in the vette, but it's significantly decreased. The acura still holds gear way better than the vette does...

And for the record, I have an automatic transmission. I am just the kind of person who manually shifts from time to time.

-Steve

Ryan77

I noticed it, and revs up to those high rpm much quicker :yesnod:

gdh

:iagree:

SLICKMAN

Me too!! :iagree:

SanDiegoPaul

The spouse has a 2000 Grand Am that somehow has NO compression braking ability

That's because the Lockup Convertor goes into the freewheel mode AFAIK

jerryp58

That's because the Lockup Convertor goes into the freewheel mode AFAIK

SanDiegoPaul,

What's the purpose of that (easier on the engine?), and what's AFAIK?

Jerry


[Modified by jerryp58, 10:01 PM 12/14/2002]

Shark Racer

AFAIK = as far as I know...

I could not imagine the T/C going into full lockup at such an RPM, so quickly. I think it has to do with the size of the engine. Most of the 4s and 6s I've driven have very negligible engine compression in relation to the vette. The engine compression on the 'vette used to make it very miserable to hold first gear through a left turn. My grandma's 3.5RL with a 3.5(surprise) liter V6 will hold well, even at a few RPM short of redline.

Take into account the smaller weight of the rotating mass, and the fact that the smaller engine has less exhaust, and the amount of compression braking decreases significantly.

-Steve

ZD75blue

Ye Olde plugged cat...

Back pressure makes the motor force the exhaust out, low back pressure pulls it out like a vacuum from the next cylinder over... I think its called exhaust scavenging.